Woven cloths, knittings, and nonwoven cloths each made of carbon fibers have been widely researched since these products are suitable for electrode substrates, and have chemical stability and an appropriate compression stress to be each applied to a gas diffusion electrode for a polymer electrolyte fuel cell.
When used for such an article, a woven cloth, knitting or nonwoven cloth made of carbon fibers is required to have gas or liquid permeability and electroconductivity that are compatible with each other at a high level. For example, Patent Document 1 discloses, in view of gas or liquid permeability, a carbon fiber sheet which is obtained by using, as an intermediate reinforcing layer for the carbon fiber sheet, a thermoplastic resin net having a low actual carbon ratio after fired, and laminating cut fibers of carbon fibers onto each other at random in a two-dimensional plane, and which has many pores of 100 to 300 μm size in the sheet that are formed by the disappearance of the region of the net after fired.
A gas diffusion electrode for a polymer electrolyte fuel cell needs to be high in electroconductivity in the sheet thickness direction thereof. For example, Patent Document 2 discloses that carbon-fiber nonwoven cloths, in each of which the axes of fibers are directed to the thickness direction of the nonwoven cloth by a needle-punching method, are expected to be smaller in electric resistance value or higher in electroconductivity than carbon fiber sheets.